Process pressure transmitters are used to monitor pressure of process fluids used in industrial processes. Process pressure transmitters include a pressure sensor that typically provides an electrical output in response to a change in process fluid pressure. Each process pressure transmitter includes transmitter electronics for receiving and processing the electrical output of the pressure sensor. The transmitter electronics are also typically configured to transmit a signal, digital, analog, or a combination thereof, over a control loop or network to a central monitoring location such as a control room.
Pressure sensors used in pressure transmitters generally include a flexible sensor element, such as an electrode plate or a piezo-resistor that deflects in response to a pressure change. The sensor element is fluidically coupled to the process fluid typically through an isolation system. The isolation system includes a metal diaphragm that is configured to contact the process fluid. The isolation system also includes a sealed passageway that extends from the isolator diaphragm to the pressure sensor. The sealed passageway is typically filled with a substantially incompressible fill fluid such as silicone oil. As the pressure of the process fluid changes, the position of the isolator diaphragm changes thereby transferring a pressure change through the isolation fluid to the pressure sensor element. When the pressure sensor element moves in response to the pressure change, a corresponding change in an electrical characteristic of the pressure sensor, such as capacitance or resistance, changes as well. The electrical characteristic of the pressure sensor element is measured by the pressure transmitter electronics and is used to compute the pressure of the process fluid.
Differential pressure transmitters are used in a variety of applications where a difference between two pressures must be measured. Examples of such applications include level measurement in a container and flow measurement across a differential pressure producer such as an orifice plate or venturi. Differential pressure sensors typically require two isolation systems to convey separate process pressures to opposite sides of a single differential pressure sensor element. Typically, a differential pressure transmitter is installed with an integral manifold/valve body that enables both zero calibration of the transmitter and removal/replacement of the transmitter without having to shut off pressure to the transmitter/manifold assembly. The interface between the transmitter and the manifold is defined by International Standard IEC 61518, entitled “Mating dimensions between differential pressure (type) measuring instruments and flanged-on shut-off devices up to 413 BAR (41.3 MPa).”
FIG. 1 is a diagrammatic view of a process fluid differential pressure transmitter coupled to a manifold assembly in accordance with the International Standard set forth above. Transmitter 10 is coupled to manifold 12 by four bolts (not shown) that extend from surface 16 of manifold 12 into transmitter 10. By using the four bolts, no fittings or additional hardware are used or required to hold the assembly together. This arrangement provides for simple assembly by the end user.
It is sometimes desirable to connect differential pressure transmitters to processes having extremely high static pressures. For example, deeply penetrating oil wells require large line pressures to transport the oil to surface levels. In applications above 413 bar, the manifolds tend to be spaced from the differential pressure transmitter with impulse piping or lines coupling the manifold to the differential pressure transmitter. This is due, at least on part, to the stresses that would be placed on the four clamping bolts if the differential pressure transmitter were bolted directly to the manifold. Given that known isolator diaphragms can exceed 0.8 inches in diameter and that two such isolators are required for differential pressure measurement, a static pressure of 10,000 psi can generate a pressure on the bolts in excess of 5,000 pounds.
FIG. 2 is a diagrammatic view of a differential pressure transmitter coupled to a manifold that is suitable for applications that exceed line pressures of 413 bar. Differential pressure transmitter 20 is coupled to manifold 22 via a pair of impulse lines 24, 26. The fluidic couplings between manifold 22 and impulse lines 24, 26 and between the impulse lines 24, 26 and differential pressure transmitter 20 are generally configured to support high line pressure. For example, such couplings sometimes use tapered fittings of the type disclosed in U.S. Pat. No. 3,362,731. However, current high pressure coupling systems require the user to route the impulse piping between the manifold and the differential pressure transmitter and to employ fittings on each end of each impulse line. Providing a high-pressure fluidic coupling system that could better accommodate coupling short runs of impulse piping would facilitate installation of differential pressure transmitters in high line pressure applications.